1. Field of the Invention
Aerogels, particularly those having porosities over 60% and densities below 0.6 g/cm.sup.3 have an extremely low thermal conductivity and for this reason are used as thermal insulating materials, as described e.g. in EP-A-0 171 722.
2. Discription of the Related Art
Aerogels in the broader sense of the term, i.e. in the sense of a "gel with air as dispersing agent," are prepared by drying a suitable gel. Understood by the term "aerogel" in this sense are aerogels considered in the narrow sense, xerogels and cryogels. A dried gel is considered an aerogel in the narrow sense of the term when the liquid of the gel is removed at temperatures above the critical temperature and starting from pressures above the critical pressure. However, if the liquid of the gel is removed under subcritical conditions, e.g. with the formation of a liquid-vapor boundary phase, then the resulting gel is designated as a xerogel. It should be noted that the gels according to the invention are aerogels in the sense of gels with air as dispersing agent.
SO.sub.2 aerogels can be prepared e.g. by acid hydrolysis of tetraethyl orthosilicate in ethanol. During the hydrolysis a gel is formed whose structure is determined, among other things, by the temperature, the pH and the duration of the gelation process. However, during the drying of the wet gel the gel structure generally collapses because the capillary forces resulting during drying are extremely great. Collapse of the gel can be prevented by carrying out the drying above the critical temperature and critical pressure of the solvent. Since in this range the liquid/gas phase boundary disappears, the capillary forces also vanish and the gel does not change during the drying process, i.e. no shrinking of the gel during drying will occur, either. Methods of preparation based on this drying technology are disclosed e.g. in EP-A-0 396 076 or WO 92/03378. However, e.g. when ethanol is used, this technique requires a temperature of about 240.degree. C. and pressures over 60 bar. Although the exchange of ethanol against CO.sub.2 before drying does reduce the drying temperature to about 30.degree. C., the pressure required is then over 70 bar.
An alternative for the above drying method is offered by a process of subcritical drying of SiO.sub.2 gels, if, before drying, the latter are reacted with a chlorine-containing silylating agent. In that case the SiO.sub.2 gel can be obtained e.g. by acid hydrolysis of tetraalkoxysilanes, preferably tetraethoxysilane (TEOS) in a suitable solvent, preferably ethanol, by means of water. In a further step, after exchange of the solvent against a suitable organic solvent, the resulting gel is reacted with a chlorine-containing silylating agent. Used as silylating agents, because of their reactivity, are preferably methylchlorosilanes (Me.sub.4-n SiCl.sub.n, with n=1 to 3). Thereupon the resulting SiO.sub.2 gel whose surface has been modified by methylsilyl groups, can be dried in air from an organic solvent. In this way aerogels having densities of less than 0.4 g/cm.sup.3 and porosities over 60% can be obtained. WO 94/25149 gives a detailed description of the method of preparation based on this drying technique.
Furthermore, before drying, the above-described gels can be treated in the aqueous alcoholic solution with tetraalkoxysilanes, and then aged, in order to increase the strength of the gel network, as disclosed e.g. in WO 92/20623.
However, the tetraalkoxysilanes used as starting materials in the above-described process are extremely expensive. Furthermore, during silylation with chlorine-containing silylating agents hydrogen chloride (HCL) and a plurality of side products associated therewith will necessarily form, which in some cases require a very expensive and cost-intensive purification of the silylated SiO.sub.2 gels by repeated washing with a suitable organic solvent. The particularly corrosion-resistant installations required in this operation are also very expensive. The safety risks associated with the formation of very large amounts of HCl gas will additionally require a very involved technique, and is thus also very cost-intensive.
A first, not inconsiderable cost reduction can be achieved by using water glass as the starting material for the preparation of the SiO.sub.2 gels. To this end, a silicic acid can be prepared from an aqueous water glass solution with the aid of ion exchanger resins, which acid will polycondense to a SiO.sub.2 gel upon the addition of a base. Then in a further step, after exchange of the aqueous medium against a suitable organic solvent, the resulting gel is reacted with a chlorine-containing silylating agent. Used as silylating agents, because of their reactivity, are preferably methylchlorosilanes (Me.sub.4-n SiCl.sub.n with n=1 to 3). The resulting SiO.sub.2 gel surface-modified with methylsilyl groups can then also be dried in air from an organic solvent. The method of preparation based on this technique is described e.g. in DE-A-43 42 548.
However the above-described problems of extremely high production costs associated with the use of chlorine-containing silylating agents are not solved by the use of water glass as starting material.
German Patent Application P 19502453.2 describes the use of a chlorine-free silylating agent. This method starts out from the silicate-type lyogel obtained with the above-described process by different methods, and reacted with a chlorine-free silylating agent. Preferably used in this case as silylating agents are methylisopropenoxysilanes (Me.sub.4-n Si(OC(CH.sub.3)CH.sub.2).sub.n with n=1 to 3). Thereupon, the thus resulting SiO.sub.2 gel surface-modified with methylsilyl groups can again be dried in air from an organic solvent.
Although the use of chlorine-free silylating agents will solve the problem of HCl formation, the chlorine-free silylating agents used also represent an extremely high cost factor.
WO 95/06617 discloses hydrophobic silicic acid aerogels obtainable by the reaction of a water glass solution with an acid at a pH of from 7.5 to 11, extensive freeing of the resulting silicic acid hydrogel from ionic components by washing with water or dilute aqueous solutions of inorganic bases, --with the pH of the hydrogel maintained in the range of 7.5 to 11--displacement of the aqueous phase contained in the hydrogel by an alcohol, and subsequent supercritical drying of the resulting alcogel.
In this process suitable alcohols for the water exchange are C.sub.1 -C.sub.5 alcohols, preferably C.sub.3 -C.sub.5 alcohols, and isopropanol in particular.
It is known that when the above-mentioned alcohols are used under supercritical conditions (WO 95/06617), esterification of the alcohols with the surface OH groups of the lyogel will take place. As a result, alkoxy-modified aerogels, e.g. isopropoxy-modified aerogels are obtained, which have hydrophobic surface groups.
However, a disadvantageous aspect of the method of preparation disclosed in WO 95/06617 is that the drying requires supercritical conditions which, e.g. for isopropanol, are at a temperature in the range of 240 to 280.degree. C. and at a pressure of about 55 to 90 bar.